The metal is much more than the water for the heavy reflector. By-pass flow should be calculated, which can optimize structure and provide input for the thermal analysis. CFD can be used for analysis the portion of metal reflector by-pass flow, and then results can be analyzed and evaluated. Finally, use the results to comparewith the theoretical analysis and experimental results. The result show that the complex channel and the characteristics of the flow field can be simulated by CFD, results are credible.